Microglial cells are the antigen presenting cells of the central nervous system and are likely involved in the initiation of multiple sclerosis (MS). The present application will explore their cell-biological and biochemical be obtained from HLA-DR2 transgenic animals and used to characterize the antigen presentation capacity of such cells, using stimulation of appropriately restricted human T cells as a readout. Two antigens will be examined in detail: myelin basic protein (MBP) and myelin/oligodendrocytes glycoprotein (MOG). Preparations of MBP and MOG obtained by in vitro transcription/translation will serve as a source of native, radiolabeled antigen, the processing of which will be monitored using biochemical techniques. These experiments will be performed not only in DR2 transgenic mice, but also in DR2-transgenics bred to homozygosity for mutations in the lysosomal proteases Cathepsin B, D, L or S. These enzymes are likely candidates for the proteolytic conversion of MBP and MOG into peptides that can bind to DR2. In this manner, the processing pathway of important autoantigens can be examined. The identification of the protease(s) essential for conversion of MBP and MOG into the offending DR2-peptide complex that initiates disease is a first step towards identification of inhibitors that could block these enzymes and thus be of therapeutic value in the treatment of ms. We shall characterize the trafficking of Class Ii molecules and antigen (MOG) in human microglia. The limited availability of this material necessarily implies a more restricted range of experiments, but the studies performed on DR2 transgenic murine microglia, the characterization of their functional properties and the assessment of the relevant proteases required for processing of MBP and MOG will allow a well-argued choice of experiments to validate the concepts elaborated for the murine model. The proposed experiments should not only uncover cell biological properties unique to microglia and essential for initiation of autoimmune T cell responses, but also point the way to new strategies for treatment of MS.